The proposed research would develop a full three-dimensional array microwave imaging system for breast imaging, with fast 3-D forward and nonlinear inverse scattering algorithms for image reconstruction. Extensive studies reveal that the electrical properties of malignant and normal mammary tissues have a large contrast. In particular, at microwave frequencies, the dielectric constant of breast tumor is about 3.5 times that in normal tissue, while this ratio is approximately 6.7 for electrical conductivity. This large contrast provides a strong scattering signal to incoming microwaves, leading to a high sensitivity of microwaves to early-stage, even small-size breast tumors undetectable by X-ray mammography. Furthermore, microwaves are a non-ionizing radiation, and the required energy is well below the safety level. This new technique is also favorable because there is no breast compression. Ongoing microwave imaging techniques using twodimensional microwave arrays have shown very encouraging results in clinical tests. Our preliminary investigation of a 2-D microwave imaging system shows promising results of images reconstructed from this experimental imaging system with nonlinear inverse scattering algorithms. This research is based on 3-D array sensors and on recent progress in computational electromagnetics that leads to fast numerical techniques. The 3-D hardware system is expected to significantly accelerate the data acquisition time. The inverse scattering algorithms unravel the complicated multiple scattering effects, leading to a high resolution in microwave image reconstruction. We have demonstrated that our algorithms can achieve a superresolution that is better than one-quarter wavelength (2.8 mm at 6 GHz). The product of our data processing is a high-resolution digital image containing the physical properties of the tissue and potential tumors. A physics-based statistical decision algorithm is applied to aid in arriving at a diagnostic result. The full three-dimensional integrated system is also expected to improve the detection of tumors in dense breasts and near the chest wall and underarm. This 3-D microwave imaging system thus combines hardware, forward simulation methods, nonlinear inverse scattering algorithms, and statistical decision algorithms for breast cancer diagnosis and screening, and has the potential to become an effective modality complementary to X-ray mammography.